The present invention relates generally to a method for placing concrete on elevated slabs and, more particularly, to a method for placing and screeding concrete on the elevated slabs which results in a substantially flat and level floor of substantially uniform thickness.
Beams and girders for supporting elevated slabs, such as floors of buildings and the like, are typically I-beams which are often pre-cambered with an upwardly curved initial form to counteract the loads that they will have to support. Corrugated sheet metal decks are then placed on the beams and girders and concrete is placed and screeded over the sheet metal to form the slab. Typically, the specified camber of the beams and girders is 80% of the deadload deflection and, for beams having a length of about thirty feet, is often in a range of approximately one inch to one and one half inches at the center of the beam, with a tolerance in a range of plus or minus approximately one-half inch. As the concrete is placed over the beams and girders, the beams and girders deflect downward under the load of the concrete. Because of the camber in the beams and the variable deflection of the beams under load, a use of known screeding systems, which are used to strike off, level and smooth the concrete, often results in a floor that is not flat, not level, and not of uniform thickness.
In order to counter the camber in the beams, concrete is often pre-placed within each bay (the area enclosed by four adjacent columns) to cause an initial downward deflection of the beams and girders. It is then possible to strike off the concrete to a uniform thickness over the sheet metal decking. However, more concrete than is actually necessary is placed at the beams to assure an adequate amount of material for strike off. The excess material may cause over-deflection in some areas, which results in low areas. As more concrete is added to fill in these low areas, further deflection may occur, which results in a slab or floor which is not flat or level.
One proposed method to obtain uniform thickness in the concrete slab is to place stands, which are fabricated metal structures that have support legs which rest on the deck and a top surface that is at the desired concrete thickness, on the metal deck. The screed then rides along the top surface of the stands, similar to a method used on slabs on grade, prior to implementation of laser screeding. The stands may later be removed before the concrete cures. Another method of obtaining a uniform thickness slab is to provide xe2x80x9cwet screedxe2x80x9d pads at a desired height above the deck followed by hand-screeding the bay or bays using the wet screed areas as a guide. The wet screed pads are made by using a handheld laser and hand trowel to strike off a roughly twelve inch diameter area of the pre-placed concrete. Two of these pads are made about ten feet apart and then a 2xc3x974 or other straight edge is used to strike off a 12xe2x80x3xc3x9710xe2x80x2 surface between the two twelve inch diameter pads. Two of these 12xe2x80x3xc3x9710xe2x80x2 struck off pads are made parallel to each other at the width of the handheld screed being used. The concrete is then struck off between these two parallel surfaces using the surface as guides for the hand held screed. While either of these methods may provide a floor or slab of generally uniform thickness, they often result in over-deflection of the beams due to excess concrete being placed in the bays prior to screeding, since the concrete must be placed high enough to assure that there is enough material before screeding.
Over-deflection of the beams results in a slab which has a lower center, such that additional concrete has to be placed in the center region to bring the slab back up to grade. This additional concrete further results in additional deflection. This is referred to as xe2x80x9cpondingxe2x80x9d in the industry and causes substantially more concrete usage and a slab that is not of uniform thickness. In many cases, ponding may result in up to 30% more concrete usage, which adds significantly to the cost of a project.
In order to counteract the ponding of the slab, 4xe2x80x3xc3x974xe2x80x3 wood shores may be placed between the beams and girders and the slab below to support the beams and girders as they are loaded. The shores have a length which provides a gap between the shores and the beams approximately equal to the initial beam camber. The shores thus limit over-deflection of the beams and girders. However, the shores interfere with the slab or area below and do not necessarily result in a floor that is flat and level. If the beams are over cambered, and the concrete weight is not sufficient to deflect the beam to a level orientation, the beams (if the bay is not pre-filled) will be continuously deflecting as the concrete is being placed, thereby resulting in non-level floors. Because the cost of correcting problems with uneven elevated slab floors after the concrete has set is high, it is highly desirable to achieve an even floor during the first placing and screeding process.
Accordingly, there is a need in the art for an improved method for placing and screeding concrete on elevated slabs, such as those supported on beams or girders. The method should account for the camber in the beams and provide a flat and level concrete floor of uniform thickness.
The present invention is intended to provide a system to compensate for beam deflection in elevated steel beams and girders while concrete is placed and screeded on slabs supported by the beams and girders. The system automatically reacts to varying loads on the beams to maintain the beams in a substantially flat and level orientation during the placing and screeding processes. This eliminates the need to pre-fill the bays and provides for the use of laser type screeding of the placed concrete, which has been proven in slab-on-grade screeding, i.e., concrete poured, smoothed and leveled on the earth or ground. Accordingly, the present invention allows for more efficient and accurate placing and screeding processes in elevated slab situations over conventional methods. Aspects of the present invention may be equally applicable on pre-cambered beams as well as on straight or level beams.
According to a first aspect of the present invention, an active shoring system for adjusting the curvature of at least one beam for supporting concrete comprises a beam monitor and a beam adjusting device. The beam monitor is operable to monitor the curvature of the beam. The beam adjusting device is operable to selectively adjust the curvature of the beam in response to the beam monitor, thereby maintaining a substantially level beam while the concrete is placed at the beam. The beam may be cambered to have an upwardly curved initial form. Preferably, the beam adjusting device initially adjusts the curvature by removing the camber in the beam prior to placing of the concrete at the beam to eliminate the need for prefilling of the area or bay associated with the beams.
In one form, the beam adjusting device is a heating device, preferably positioned along a lower flange of the beam, which at least initially heats the beam to reduce the curvature of the beam. In like manner, the upper flange of the beam could be cooled to obtain the same effect. A cooling fan may be provided which is operable to cool the beam in response to the beam monitor detecting a generally level orientation of the beam. In another form, the beam adjusting device comprises an adjustably weighted container, the weight of which may be increased or decreased to adjust the curvature of the beam in response to the beam monitor. In yet another form, the beam adjusting device may comprise a generally horizontally mounted hydraulic cylinder or jack secured to opposite ends of each beam, such that extension and retraction of the hydraulic cylinder causes a decrease or increase in the curvature of the beam.
According to another aspect of the present invention, a method for actively adjusting a curvature of at least one beam comprises the step of providing at least one beam and placing concrete for support by the beam. The curvature of the beam is then monitored with a measuring device. The curvature is then automatically adjusted in response to the measuring device, in order to maintain a generally level orientation of the beam while the concrete is placed at the beam. In one form, the beam is provided with a camber, such that the beam has an initial upward curvature therealong. Preferably, the curvature of the beam is initially reduced such that the beam is at a generally level orientation prior to the step of placing concrete at the beam.
Therefore, the present invention provides an active shoring system which maintains a substantially level orientation of the beams and girders of a bay area of a structure or building while concrete is being placed, screeded and/or cured at the bay region. This is accomplished without requiring pre-filling of concrete at the bay, since the beams and girders may be straightened or leveled prior to placing concrete at the area above them. This results in floors which are substantially flat, level, and of uniform thickness throughout. The present invention thus substantially reduces the likelihood of ponding and of waste and/or repair of the concrete slabs. Therefore, the present invention provides a system and method for allowing the placing and screeding of concrete at elevated slabs which is more efficient and produces a flatter, higher quality floor over conventional processes.
These and other objects, advantages, purposes and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.